This invention relates to sensors for use in the surface measurement of a partial pressure of a gas in the blood of a patient by use of polargraphic cells or sensors. In the prior art such sensors have been known for several years and it has been determined that in vivo or surface measurements utilizing these devices produce results which are a function both of the partial pressure of the gas in the blood and of the local blood circulation in the region immediately adjacent to the area where the measurement is taken. Since this area is generally very small, typically 0.5 square centimeters or less, it has been found practical to hyperaemise the skin and the subjacent tissue as a method of reducing the dependence of the sensor on the local blood circulation and to thereby produce a more accurate reading of the true partial pressure of the measured gas in the blood. While this may be accomplished by chemical methods, using vasodilating compounds such as histamine, nicotinic acid, etc., the same result has been more conveniently accomplished by the application of heat to the skin, which also produces local hyperaemisation.
Prior devices which use heat to hyperaemise the skin suffer from several shortcomings to which the invention herein is directed. Initially, heat is applied from within the sensing cell in such devices and passes heat directly through the sensing face of the sensing cell to the skin. This is the method, for example, of Eberhard et al, U.S. Pat. No. 3,795,239. This direct heating of the sensor membrane and electrolyte adjacent to the membrane decreases the useful life of the sensor and results in increased cost to the user both directly by making necessary the purchase of extra sensor units and indirectly by consuming personnel time to replace the in vivo sensor. A principle result of the current invention is improved economy through the superior heater arrangement resulting in longer equipment lifetime without a significant reduction of the hyperaemisation in a region subjacent to the sensor.
A second problem in the current art has been an observed downward drift in readings by the instruments attached to the transcutaneous sensor in the absence of any real reduction in arterial blood gas level as measured directly. The origin of this long term drift is not precisely known, although the configuration of the instant invention has been observed to have significantly less drift. It is hypothesized that the drift is due to local edema in the immediate vicinity of the sensor membrane caused by the application of heat over prolonged periods of time and the associated hyperaemisation immediately below the sensor membrane. The instant invention has a remarkedly reduced instrument drift thought to be the result of the avoidance of directly heating the skin beneath the sensor membrane and thus not subjecting that skin area to welting or other effects which commonly occur after the prolonged application of heat to the skin surface.
An additional result of the present invention is the obtaining of accurate readings at slightly lower average temperatures than is currently practiced. The present invention will have as its principle application the measuring of transcutaneous oxygen levels in prematurely born children. The constant application of heat to their skin creates a blistering problem. Current devices operate at a heater temperature of 45.degree. C. and a skin temperature of 43.degree. C., whereas the present invention works reliably at a temperature of about 43.degree. C. and a skin temperature of 41.degree. C. This small reduction in temperature helps to guard against blistering.
The present invention also provides an improved and economical method of measuring the blood perfusion in the area of the sensor membrane by use of a differential temperature measurement. Previously a local circulatory failure in the region of the sensor was only determined by measuring the energy consumption necessary to maintain the heater at a constant temperature. In the event of a circulatory failure, either locally at the membrane measurement site or a general failure, the cooling effect of blood flowing through the subjacent skin would decrease, resulting in decreased energy consumption of the heater. This energy consumption would be monitored as a means of detecting the circulation failure, but has certain drawbacks. It is not responsive directly to the blood circulation and perfusion near the membrane, but rather is a measure of all of the cooling elements around the heater, only one of which is the skin. Thus, this method is responsive to the thermal capacity of the heater and total environment surrounding the heater, only one portion of which is the cooling effect of the blood perfusion under the skin. Finally, it should be noted that the present invention is especially adaptable for use with disposable cartridge sensors of the type described in U.S. Pat. No. 3,826,730 of Watanabe and Leonard. However, the principles of this invention could be practiced with other sensor cells with minor structural modifications.
In the prior art it has been common to utilize electric heating wire for heating sensors to raise the skin temperature. Such heating coils generally create temperature differentials within the sensor itself, often generating hot spots on the sensor surface which damage or blister the patient's skin. This is caused to a great extent by the inability of the small sensor body to properly distribute the heat generated by the heating coil throughout the sensor structure.